Nonvolatile memory devices have been provided by the use of programmable-resistance materials based on mobile metal ions embedded in an electrolyte glass matrix. The according technology is known under the terms conductive-bridging RAM (CBRAM) or programmable metallization cell (PMC). Electrical pulses switch the resistance of these programmable-resistance materials reversibly and persistently between a low-resistance state and a high-resistance state. It is believed that this resistance switching is caused by electrochemical reduction and oxidation of the metal ions in the glass which leads to creation and annihilation of a metallic protrusion that forms a bridge between the two electrodes (some authors refer to this protrusion as “dendrite”; the teaching of the present text does not depend on a particular interpretation of the electrochemical processes or the shape of arising structures).
It has also been proposed to use programmable-resistance materials based on mobile metal ions embedded in an electrolyte glass matrix in select elements for nonvolatile memory cells. Select elements are required to make sure that during a read-, write- or erase-process of a certain memory element, memory cells of the same word or bit line do not interfere. The use of programmable-resistance materials for select elements has for example been proposed and described in U.S. Pat. No. 7,382,647.
Also in select elements, electrical pulses can switch the resistance of the programmable resistance material from a high-resistance state to a low-resistance state. In the select elements, the metallic protrusion that is believed to form, during application of a switching pulse, may according to a first option be annihilated after disconnecting the voltage supply due to diffusion, and the element returns to its high-resistance state, i.e. the select element is volatile. According to a second option, the metallic protrusion may, like in a memory cell, be annihilated by a voltage pulse of opposite polarity.
When used as a nonvolatile memory cell, these electrolyte elements have a large variation of the operating parameters, especially the resistance of the low-resistance state. The large variations of the properties of nominally identical programmable resistors used in the memory cells are severe drawbacks. Further drawbacks are the limited retention time, particularly at elevated temperatures, and the limited endurance.
Also when the electrolyte element is used as a select element, the limited endurance is a drawback. The failure mechanism is over-writing of the select element when writing the nonvolatile memory cell: To set, for example, phase-change RAM cells or also resistive memory cells to the low-resistance state, a long electrical pulse is required. By this, the metallic protrusion in the select element grows so extensively that it cannot be annihilated anymore.